It is believed that the earliest studies of scour occurred during the early 19th century by railroad engineers. These were probably unscientific and sporadic observations at best. By the turn of the century, hydraulic engineers became involved in laboratory studies of pier scour, but the main responsibilities for field studies were borne largely by the railways. In a number of foreign countries this tradition remains even today.
Scour is the result of the erosive action of flowing water, excavating and carrying away material from the bed and banks of streams. Naturally, not all materials scour at the same rate, which makes the predictability of scour very difficult. This phenomena typically occurs in stream and river bottoms during periods such as heavy rainfalls and spring runoffs. The result can sometimes be cavities having depths twice that of the water depth. Unfortunately, while the evidence of scour is obvious during its occurrence, as water levels recede and flow conditions return to normal these cavities often refill making detection difficult. These difficulties in prediction, and late detection of scour have led to the necessity of monitoring sites for the occurrence of scour.
Equipment used for scour observations has usually been quite simple: sounding rods for shallow flows and lead sounding weights on a line for deeper flows. Both of these devices were developed to sound for navigation depths hundreds of years ago, and were adapted for depth soundings in connection with stream flow measurements during the 19th century. The main adaptations involved streamlining the sounding weights and using stay lines or vertically supported sounding rods so that the weights or rods would not be swept downstream in high velocities. Early stream flow measurements were often made at ferry crossings, and special supports were designed to mount on the ferry cable so that the sounding rod with the meter could be held vertically approximately three feet in front of the bow of the ferry.
Eventually, these crude devices gave way to more advanced equipment. In the 1950's significant advances were made in sonar, sonic sounders, electronic positioning equipment, and radar. There were problems with these techniques, however. Accuracy, flow depth restrictions, and perhaps most importantly since many existing bridges are maintained by small local authorities, high cost--all acted as significant disadvantages of these devices. For example, sediment concentration along the Yellow River is so high that the standard sonic sounders cannot distinguish between the moving sediment and the non-moving bed. U.S. Pat. No. 4,855,966 to Cinquino shows one particular sonic device wherein a housed probe descends within a scour hole, and a bridge mounted sonic transducer operates to determine the distance of the probe from a reference point on the bridge.
Another technique, as shown in U.K. Patent Application No. GB 2245736 to Waters, uses buried omni-directional mercury switches. As scour occurs and the switches are unburied, the water flow causes these switches--of which there are only three--to actuate sending a signal to a bridge mounted monitor. The installation of this device is very delicate and cannot be merely driven into the stream bed like the present invention. Instead, a large area of the bed must be dug up, and then refilled after insertion of the Waters' unit. Disturbing the stream bed in this fashion actually increases the area's susceptibility to scour. The Waters device is also relatively expensive due in part to the installation process.
Still another device, known only to have been used in New Zealand as long ago as 1982, employed a radioactive source housed within a large lead weight. This device is described in the printed publication entitled "Field Measurement Of Scour Depth Using A Portable Gamma Spectrometer" by B. W. Melville, and submitted as Report No. 287 to the Department of Civil Engineering at the University of Auckland, in Auckland, New Zealand. In Melville's device a gamma-ray probe was used to determine the location of a radioactive material (Cobalt 60) contained within the lead weight as it sank into a scour hole. It is believed that the general U.S. population would look unfavorably on the use of radioactive material in their various water supplies.
In preparation for development of the present invention, criteria needed to be established for defining effective scour monitoring devices. They include: 1) a capability for installation on or near a bridge pier or abutment; 2) an ability to measure maximum scour depth within an accuracy of +0.5 feet; 3) an ability to obtain scour depth readings from above the water or from a remote site; and 4) an ability to operate during storm and flood conditions. Other desirable criteria were also established, and include: 1) a capability to be installed on most existing bridges or during construction of new bridges; 2) a capability to operate in a range of flow conditions; 3) a capability to withstand ice and debris; 4) relatively low cost; 5) vandal resistant; and 6) an ability to be operated and maintained by highway maintenance personnel. Until the present invention, no scour monitoring device has met each of these criteria. The present invention meets or exceeds each of these criteria.
The present invention, in both its apparatus and methods, recognizes and addresses these criteria and overcomes the limitations perceived by those skilled in the art by presenting a design which, among other aspects, allows for installation near a bridge pier or abutment on existing bridges as well as new constructions. Those skilled in the art of scour monitoring device design have long been aware of the problems of accuracy, high cost, remote operation, and maintenance or repair. Millions of dollars have been spent to date in research by those using sonic sounders, sonar, electronic positioning equipment, and radar, All the while the necessary arts and elements for implementing the disclosed invention have existed for sometime. The various patents cited show substantial attempts by those skilled in the field to meet a few of the above criteria, but they have failed to create a reliable instrument which meets and exceeds all the required and desired criteria. Some have been able to install upon new and existing bridges, others have discussed remote data collection, and still others have addressed the accuracy concerns. However, a single system which integrates these capabilities into a reliable inexpensive scour monitoring device has not existed until the present invention. Instead of understanding the true problems, manufacturers have coped with the inherent limitation to some of these devices and methods. There appeared to be a failure to fully understand the problems and impacts of properly monitoring bridge scour.